Advanced woody defense quiz 3

Co-evolution

When two or more species impact each other’s evolution

Evolutionary arms race theory

organisms must constantly adapt and evolve to gain reproductive advantages and survive while facing opposing organisms that are also constantly evolving and changing.

Gene-for-Gene relationship

For each gene controlling resistance in the host, there is a corresponding gene controlling avirulence in the pathogen

Paleozoic era plant defense summary

At the beginning of the era plants were herbivore free and didn’t require defenses.

At the end of the permian period vertebrates appear and start feeding on plants.

Mesozoic era plant defense summary

Plants have lower productivity

Large herbivorous dinosaurs

Cenozoic

greater co-evolution between between plants and herbivores
organ level herbivory appears

Gymnosperm evolution

• long-lived leaves/needles

• favor defenses

• greater anatomical defenses (constitutive)

• longer co-evolution with herbivores

• wind pollinated

Angiosperm evolution

• Short-lived leaves

• Favored conditions and growth

• More specialized chemical defeneses

• Rapid diversification and shorter co-evolution with herbivores

• Diverse pollination methods

Pathogen infection mechanisms

• chemical substances that lead to infection:

  • enzymes

  • toxins

  • growth regulators

  • polysaccaharides

Pathogen enzymes examples

• lipases

• cutinases

Lipases

breakdown fats and oils

Cutinases

• breakdown cutin polyesters

  • breach the wax and cuticle of the plant cell wall

  • breaks down the structure of host cells

  • disintegrates intercellular food substances

  • interferes with membrane components

Type of pathogenic enzyme produced depends on

The life cycle of the fungus T

The goal of pathogenic enzymes is

Not trigger plant responses

Fungal infection steps

• Spore lands and adheres to the cuticle

  • consitutively releases enzymes

• Spore germination on the host plant cuticle

  • Induced enzymes are released

• Spore germ tube elongation and formation of penetration structures

  • mechanical penetration

• Penetration of the host plant cuticle and colonization of inner tissues

  • colonization

Pathogenic toxins

• interfere with cellular activity

• are effector proteins

Examples of pathogenic toxins

NLPs

Chitin-binding lysin motif (LYSM)

NLPs

• necrosis and ethylene inducing like proteins

• phytotoxic activity

• induce cell death

• induce unwanted plant immune response

Chitin-binding lysin motif (LysM)

• binds to pathogen’s chitin to eliminate it from the infection site

• prevents the host from activating immune receptors

Polysaccharides

• passively interrupt water translocation in plants

• wilt-inducing factors

Growth Regulators

pathogens synthesize plant hormones to disrupt signaling pathways and increase their virulence

Examples of growth regulators

• coronatine

• cytokinin molecules

• auxin

• Lasiojasmonate A

• abscisic acid

Coronatine

• mimics JA and suppresses SA

Cytokinin molecules

• regulate plant growth & development

• modulate plant resistance

Auxin

• growth and development

Lasiojasmonate A

activates JA regulated processes

Abscisic acid

abiotic stress response → antagonizes SA and JA responses

Overcoming Plant immune pathways

• inactivating JA pathway

• Suppression of RNA silencing

How fungi inactivate JA pathway

• Fungi secrete enzymes that hydroxylates plant’s JA → Monooxygenase (Abm)

  • triggers JA production to stop

RNA silencing

• AKA RNA interference (RNAi)

• done by small-interfering RNA (siRNA) → impacts gene expression

• acts as a antiviral mechanism

Viral Suppression of RNA silencing

• done by VSRs - viral suppressors of RNA

• protein and RNA based suppressors

Pathogen suppression of RNA silencing

Bacteria - auxin silencing

Oomycetes/fungi - alter host miRNA and siRNA

Chemical counter defense in insects depends on

what the insect is trying to accomplish

Methods of counter defense in insects

• avoidance

• excretion

• sequestration

Chemical avoidance in insects

• depends on feeding strategies

• oviposition behaviors - females don’t lay eggs on unsuitable plants

• insects seek plants whose toxins are distasteful to outside predators

• total avoidance of plans with bitter tasting chemicals

Neutralizing plant defenses

• secrete detoxifying enzymes

• using plant metabolites as feeding and oviposition stimulates

• create detoxifying enzymes

• Decrease enzyme activity of beta-glucosidases (involved in lignification)

• sequester plant defenses to work for insects

  • salicylaldehyde → deters predators

Plant defenses can be sequestered and used as

• Defenses against predators

• Pigment

• Pheromones

Insect metabolic resistance

• biotransformation of plant toxins due to production of detoxifying enzymes

• Three major super-families of enzymes:

  • Cytochrome P450 monooxygenases (CYP genes)

  • Glutathione S-transferases

  • Carboxylesterases

Symbiotic Relationships (beetles & fungi)

• bark beetles + filamentous fungi

• ambrosia beetles + fungi → farm fungi

Beetles are able to feed on fungi and transport them to new hosts.

Fungi overcome tree defenses and turn metabolites into pheromones to attract more beetles

Symbiotic relationships (bacteria and fungi)

• bacteria degrade anatomical defenses, fix nitrogen, and degrade monoterpenes

• produce antibiotics against competing fungi

Invasive species success is determined by

A species range

Traits related to arrival

Traits related to establishment

A species range:

1. spatial distribution of the habitat

2. accessibility of habitats suitable for existing populations

Helpful traits related to arrival (invasives)

• naturally mobile

• affinity for humans and their products

• resistant or dormant life stages

Helpful traits related to establishment (invasives)

• high reproductive rate

• wide host preference

• tolerant of climate extremes

• efficient mate location

• high genetic or phenotypic plasticity

  • can change in response to a stimuli

Outbreak

the occurrence of infection or infestation in excess of normal expectancy

Invasive species “Enemy-free space hypothesis”

• lack of predators reduce or eliminates a species’ vulnerability

  • lack of biological control

• top down approach for control

Invasive species “Defense-free space hypothesis”

• hosts are more susceptible due to a lack of co-evolution defense history with invasive

• take a bottom up approach for control, try to create resistance

Chestnut case study - enemy free hypothesis

• no fungal viruses in US to combat fungus (hypoviruses)

Chestnut case study - defense free hypothesis

• American chestnut lacks the genes or the responses for gene upregulation to defend against the blight

• American chestnut responses differ from Asian chestnut responses to infect and are less effective

• C. parasitica (blight) has virulence factors that can overcome existing defenses in American chestnut

Hemlock wooly adelgid case study - defense free hypothesis

• eastern hemlock has more palatable leaves caused by differences in foliar chemistry compared to asian varieties

  • eastern high levels of N and K

  • Asian high levels of Ca and P + increased terpenes

Hemlock wooly adelgid case study - enemy free hypothesis

• lack of wooly adelgid predators in the US - generalists are less likely to eat them

• more natural enemies in their native range

Key take-aways for invasives

• Global trade has facilitated the rapid introduction of nonnative species across the globe

• Successful invasive species possess traits that allow them to spread rapidly and colonize native tree species

• Native species lack the evolutionary relationship with nonnative species leading to defense-free space

Chinese chestnut defenses against blight

• Entire JA metabolic pathways was activated

• JA, SA, ET, auxin pathways positively modulate chestnut resistance

• MAPK cascade also upregulated

  • mitogen activated protein kinase

• upregulated genes that produce chitinase

• Oxaloacetate acetylhydrolase gene - helps reduce the oxalate produced by fungi